1. Field of the Invention
The present invention relates generally to an Automatic Re-transmission Request (ARQ) method in a wireless communication system, and in particular, to an ARQ method in a Broadband Wireless Access (BWA) system.
2. Description of the Related Art
Wireless communication systems have been developed to accommodate more users by multiple access schemes. A major multiple access scheme is Code Division Multiple Access (CDMA). CDMA, originally proposed for voice communication, has now evolved to additionally process high-speed data. The driving force behind the development of the CDMA technology is user demands for high-speed data transmission and rapid technology development. At present, most of 3rd generation (3G) mobile communication standards have been approved and commercially deployed.
However, CDMA faces limits in high-speed data transmission due to limited resources. Nonetheless, a user demand for higher data rate is increasing.
In this context, extensive research is being conducted and attempts have been made with the aim to transmit data at higher rates in the wireless communication field.
Among these attempts, BWA is an actively studied area. Particularly, Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) were introduced for BWA incorporation. While OFDM is a technology considered for the future-generation broadcasting service in Europe, it is being applied to cellular systems, for the purpose of transmitting a large amount of data at high rates to more users. OFDMA is a transmission scheme in which a plurality of channels are established using orthogonal frequencies and one or more channels are allocated to each user. OFDMA was standardized as the Institute for Electrical and Electronic Engineers (IEEE) 802.16d and the IEEE 802.16e standards.
Typically, communication systems must guarantee the safety of data transmission. The Safety is a more significant factor to wireless communications than to wired communications. The safety refers to the transmission of data with integrity, that is, without loss. Many methods are used to prevent data loss in the communication systems. A main approach is ARQ. In ARQ, when a receiver fails to receive or decode data transmitted from a transmitter, the receiver notifies the transmitter of the reception failure so that the transmitter can retransmit the data.
A description will be made of an ARQ scheme in an OFDMA system compliant with the IEEE 802.16d and the IEEE 802.16e standards.
According to the IEEE 802.16 standards, the transmitter segments a Medium Access Control (MAC) Service Data Unit (SDU) into ARQ blocks, for transmission. The receiver notifies the transmitter if the individual ARQ blocks have been received successfully. Both the transmitter and the receiver must identify the transmitted blocks to determine which block is received and for which block an ARQ is to be created. For block identification, each block is labeled with a Block Sequence Number (BSN).
When BSN information is matched between the transmitter and the receiver within a predetermined period of time, an ARQ window is moved. However, a BSN mismatch may occur between the transmitter and the receiver. In this case, a typical wireless communication system determines that an error has occurred to an ARQ transmission and initializes the ARQ transmission. This is called an ARQ Reset.
The IEEE 802.16 standards specify that an ARQ Reset takes place by transmitting/receiving an ARQ Reset message between the transmitter and the receiver if time intervals ARQ_TX_WINDOW_START and ARQ_RX_WINDOW_START do not increase when ARQ_SYNC_LOSS_TIMEOUT has elapsed.
For the ARQ Reset, activation and initialization of an ARQ_SYNC_LOSS_TIMEOUT timer is to be specified.
Yet, how the ARQ_SYNC_LOSS_TIMEOUT timer is initialized is not clearly set forth the standard. In relation to the time to activate the ARQ_SYNC_LOSS_TIMEOUT timer, the standards define ARQ_SYNC_LOSS_TIMEOUT as “the maximum time interval ARQ_TX_WINDOW_START or ARQ_RX_WINDOW_START shall be allowed to remain at the same value before declaring a loss of synchronization of the sender and receiver state machines when data transfer is known to be active. The ARQ receiver and transmitter state machines manage independent timers. Each has its own criteria for determining when data transfer is ‘active’”.
The above description, which defines ARQ_SYNC_LOSS_TIMEOUT simply as the time interval for which ARQ_TX_WINDOW_START or ARQ_RX_WINDOW_START can be kept unchanged when data transfer is known to be active, does not provide an explicit specification of ARQ_SYNC_LOSS_TIMEOUT.
In addition, since the cause of the ARQ Reset is confined to the ARQ_SYNC_LOSS_TIMEOUT timer, abnormalities that may be created during an ARQ process are not adequately provided for. Hence, a need exists for restricting an unnecessary ARQ Reset by clarifying the ARQ_SYNC_LOSS_TIMEOUT operation, and defining additional causes for the ARQ Reset, for an efficient ARQ Reset.
FIG. 1 is a diagram illustrating a signal flow referred to for describing expiration of the time interval ARQ_SYNC_LOSS_TIMEOUT according to the IEEE 802.16 standard.
Before describing FIG. 1, it is made clear that transmission and reception take place in the MAC layer 100 of a transmitter and the MAC layer 110 of a receiver and thus the MAC layers 100 and 110 are referred to as a transmitter 100 and a receiver 110, respectively.
Referring to FIG. 1, upon receipt of a MAC SDU from an upper layer in step 120, the transmitter 100 activates an ARQ_SYNC_LOSS_TMEOUT timer in step 122. This timer is used to detect an ARQ sync loss timeout. In step 124, the transmitter 100 transmits a MAC PDU created from the MAC SDU to the receiver 110. The receiver 110 then receives the MAC PDU and activates an ARQ_SYNC_LOSS_TMEOUT timer in step 126. The receiver 110 decodes the MAC PDU and notifies the transmitter 100 of the decoding result. In the case illustrated in FIG. 1, it will be assumed that the decoding failed.
The receiver 110 transmits a Negative-ACKnowledgement (NACK) signal requesting a retransmission to the transmitter 100 in step 128. The transmitter 100 retransmits the MAC PDU to the receiver 110 in step 130. The MAC PDU can be data with the same bits as those of the initially transmitted data or redundancy data with which to help decoding the initially transmitted data.
The receiver 110 decodes the retransmitted MAC PDU in step 132 and notifies the transmitter 100 of the decoding result in step 134. There it will be assumed that the retransmitted MAC PDU is successfully received and decoded. The receiver 110 transmits an ACKnowledgement (ACK) signal to the transmitter 100 in step 134.
In step 136, the receiver 110 transmits the MAC PDU to an upper layer. This is a normal traffic transmission procedure between the transmitter 100 and the receiver 110.
However, an ARQ sync loss timeout may occur even in the absence of any traffic data to transmit from the transmitter in the normal traffic transmission procedure.
With regard to this problem, the conventional technology according to the standards does not provide for a method to temporarily stop the ARQ_SYNC_LOSS_TIMEOUT timer. As in steps 140a and 140b of FIG. 1, even in the absence of any traffic to be transmitted from the transmitter 100, the ARQ_SYNC_LOSS_TIMEOUT timer expires. As a result, an ARQ Reset is carried out in step 142.
With the ARQ Reset, the transmitter 100 and the receiver 110 reactivate their ARQ_SYNC_LOSS_TIMEOUT timers in steps 144a and 144b. If no traffic still exists, the ARQ sync loss timeout lasts till steps 146a and 146b. Thus, the transmitter 100 and the receiver 110 carry out the ARQ Reset again in step 150. They also reactivate the ARQ_SYNC_LOSS_TIMEOUT timers in step 150a and 150b. 
The repetition of the ARQ Reset and ARQ_SYNC_LOSS_TIMEOUT timer activation causes the following problem.
Even though ARQ_TX_WINDOW_START and ARQ_TX_WINDOW_START are not updated due to the absence of transmission traffic, the ARQ Reset is unnecessarily repeated. In other words, step 150 of FIG. 1 is performed.
In a real implementation, ARQ_SYNC_LOSS_TIMEOUT is set to 655350 μsec. If there is no traffic to be transmitted within the 655350 μsec, an ARQ Reset message is transmitted and received and an initialization is performed, despite synchronization between the transmitter and the receiver. The unnecessary ARQ Reset leads to channel resource consumption. If a terminal uses a portable power supply like a battery, the standby time or call time of the terminal is reduced.
Now a description will be made of the cause of the above-described additional ARQ Reset. First, an overview of the operations of the ARQ transmitter and receiver will be provided.
The receiver transmits the BSN of a received ARQ block to tell which ARQ block has been received and an ACK/NACK signal for the ARQ block to the transmitter. The transmitter determines if the BSN falls within an ARQ window size or range. If the BSN falls within the ARQ window size and thus it is valid, the transmitter processes the ACK/NACK signal.
Although the transmitter is required to retransmit an ARQ block if the receiver wants, if the BSN is beyond the ARQ window range, the transmitter neglects the ACK/NACK signal. This will be described in great detail with reference to FIG. 2.
FIG. 2 is a diagram illustrating a signal flow for an ARQ Reset when a traffic transmission error occurs between the transmitter and the receiver according to the IEEE 802.16 standards.
Referring to FIG. 2, upon receipt of a MAC SDU from the upper layer in step 200, the transmitter 100 activates the ARQ_SYNC_LOSS_TIMEOUT timer in step 202. As stated before, this timer is used to detect an ARQ sync loss timeout. In step 222, the transmitter 100 generates a MAC PDU with BSNs of 0 to 4 and transmits it to the receiver 110.
In step 224, the receiver 110 receives the traffic with the incorrect BSNs due to errors, and transmits an ACK with the incorrect BSNs to the transmitter 100 in the illustrated case of FIG. 2. For example, although the transmitter 100 transmits a MAC PDU with the BSNs of 0 to 4, the receiver 110 determines the BSNs to be 8 to 12 due to the errors and transmits an ACK signal for the traffic with the incorrect BSNs of 8 to 12 to the transmitter 100.
Because of the incorrect BSNs, the transmitter 100 neglects the ACK signal in step 226. In step 228, as an RX_PURGE_TIMEOUT timer expires, the receiver 110 considers that reception of the MAC PDU with the BSNs of 8 to 12 is completed and provides the data with the incorrect BSNs to the upper layer.
In the above conventional technology, the receiver 110 transmits to the transmitter 100 an ACK signal with incorrect BSNs due to errors. Later, the receiver 110 fails to receive necessary ARQ blocks and discards them as the RX_PURGE_TIMEOUT timer expires. Nevertheless, the transmitter 100 does not recognize that the errors have occurred and transmits a MAC PDU with BSNs following those of the MAC PDU transmitted in step 222, when an ARQ_RETRY_TIMEOUT timer expires.
As described above, in the case where the receiver 110 transmits to the transmitter 100 an ACK for an ARQ block with a BSN falling within an ARQ window but not transmitted by the transmitter 100, the transmitter 100 has no action to take to deal with this situation.